Global transport will continue to be powered largely by petroleum-based liquid fuels in the next few decades. The increase in demand for transportation fuel will be mostly in the commercial transport sector (heavy duty road, air, marine and rail), and much more diesel and jet fuel will be needed in the future compared to gasoline. Moreover, gasoline octane quality needs to increase to enable more efficient spark ignition engines. This poses significant challenges to the refining industry and is likely to increase the availability of low octane components in the gasoline boiling range.
One approach to mitigate the imbalance in demand growth between gasoline and middle distillates is to change the demand requirements of future engines. There is little prospect of moving aviation from using conventional jet fuel to any significant degree, but some compression ignition (CI) engines could move from conventional diesel to natural gas (NG), particularly in markets such as the U.S. where the shale gas revolution has brought in an era of cheap and abundant gas.
Development trends in internal combustion (IC) engines are likely to have a significant impact on the properties required of future fuels. Moreover, there are very good opportunities to develop new engine/fuel systems which can be beneficial from the fuel and engine side as well as help to mitigate the demand imbalance. For instance, CI engines running on low octane (research octane number (RON) of about 70) gasoline and with more relaxed volatility requirements compared to current gasoline could be at least as efficient as current diesel engines but cheaper. This concept is known as Gasoline Compression Ignition (GCI).
Today's IC engines are either spark ignition (SI) engines running on gasoline or CI engines running on diesel fuel. In SI engines, gasoline is premixed with air, and the mixture is compressed. Heat release occurs in an expanding turbulent flame initiated by a spark near the top of the compression stroke, top dead center (TDC). In a diesel engine, air is compressed, and fuel is injected near TDC. Combustion is initiated by autoignition as the fuel vaporizes, mixes, and reacts with oxygen in the engine cylinder.
Currently, the passenger car sector is dominated by SI engines, whereas the commercial sector (heavy duty road, air, rail and marine) is dominated by CI engines.
SI engines are run at a stoichiometric fuel and air mixture that enables a three-way catalyst to be used to treat the exhaust and reduce tail-pipe emissions of unburned hydrocarbons (HC), carbon monoxide (CO) and NOx to extremely low levels.
CI engines are more efficient than SI engines, but suffer from high engine-out emissions of particulates and NOx (nitrogen oxides) which are very difficult to control by after-treatment of the oxygen-rich exhaust with catalysts. Particulates are formed in diesel engines because diesel fuel with a Cetane Number (CN) greater than about 40 autoignites too easily, before it has had a chance to mix adequately with oxygen. Exhaust gas recirculation (EGR) can be used to control NOx, but it reduces in-cylinder particulate oxidation and leads to increased engine-out particulate emissions. Hence, it is very difficult to control particulates and NOx simultaneously in diesel engines.
Engine manufacturers are currently trying to solve this problem by using very expensive technology such as very high injection pressures and complex after-treatment systems to control NOx and particulates. Some of these technologies compromise fuel efficiency.
Controlling particulates and nitrogen oxides (NOx) at reasonable cost without compromising efficiency would be much easier to meet if diesel engines were run on fuels which do not ignite as easily as diesel fuel, allowing more time for fuel and air to mix before combustion starts.
Gasoline Compression Ignition (GCI) engines are currently under development that can achieve substantially higher combustion efficiency than conventional diesel and gasoline spark ignition engines. These new GCI engines are similar in operation to Homogeneous Charge Compression Ignition (HCCI) engines, but have the fuel injection just prior to TDC on the compression stroke. These new engines can utilize different fuel compositions than conventional internal combustion engines. GCI engines may be at least as efficient as CI engines, and more efficient than the most advanced SI engines. They may also be less expensive than advanced CI engines.
There is a need for a fuel which has acceptable particulate and NOx emissions and which can be used in GCI engines, and for methods of making the fuel.